Radiators and filter screens in the cooling system of farming and forestry vehicles become blocked with debris such as bits of straw in the course of normal use necessitating, in a former state of the art, frequent work-stoppage interruptions for cleaning away the debris, otherwise risking damage to the engine through overheating. Various solutions involving a reversal in air flow have been proposed to alleviate this problem. One such solution is to change the direction of rotation of a ventilating fan. This, however, requires an inevitably expensive reversible transmission. Another solution comprising a reorientation of fan blades becomes increasingly difficult to technically implement as increasing numbers of fans are concerned. Hinged ventilator flaps represent yet another solution. With the direction of fan rotation remaining the same, the flaps are alternately opened and closed to redirect air drawn by the blowers. The auxiliary equipment necessary to operate the flaps is generally disturbance-prone and space-consuming, while sealing problems lead to relatively high air leakage. An important disadvantage of the hinged-flaps solution is that it cannot be universally utilized.